1. Field of the Invention
The field of the invention relates generally to organic light emitting diode (OLED) devices, and more particularly to a cathode structure in a flexible OLED device.
2. Description of Related Art
Organic electroluminescent devices, such as organic light emitting diodes (OLEDs), have been widely used for display applications, and the use of such devices in general lighting applications is gaining acceptance. An OLED device includes one or more organic light emitting layers disposed between two electrodes, e.g., a cathode and an anode, formed on a substrate. An encapsulating cover is disposed over the cathode. The OLED device may “top-emitting”, wherein the produced light is emitted through the cover, or “bottom-emitting” wherein the produced light is emitted through the substrate. The organic light emitting layer emits light upon application of a voltage across the anode and cathode, whereby electrons are directly injected into the organic layer from the cathode, and holes are directly injected into the organic layer from the anode. The electrons and the holes travel through the organic layer until they recombine at a luminescent center. This recombination process results in the emission of a photon, i.e., light.
Large area OLED devices typically combine many individual OLED elements on a single substrate. Use of large area OLED devices as a light source in lighting fixtures is gaining acceptance in the lighting industry. OLED devices, which typically have an Al/Ag cathode structure and a thickness of less than 200 nm, are an efficient, high-brightness light source, but are not without certain inherent drawbacks. The devices generate significant internal heat, which can be dissipated in larger area devices, but also operate in high temperature environments. Prolonged exposure to high temperatures may induce localized degradation of the devices (e.g., de-lamination of the light-emitting layers), often resulting in color shift and/or highly visible dark spots in the illumination field. High temperatures also result in an overall decrease in brightness of the device, thus limiting the useful life of the devices.
One approach to improving heat management and cooling an OLED device is set forth in published U.S. Pat. Application No. 2005/0285518, which proposes a “thick” cathode configuration. The cathode has a continuous thickness of greater than 500 nm over and between the light emitting elements, and is preferably greater than 10 microns. The '518 publication also proposes to add a heat conductive layer to the cathode cover, with this layer preferably having a thickness of at least 100 microns. A premise of the '518 publication is that a cathode below 500 nm thickness will not provide sufficient heat conductivity, and that a “thick” cathode and thermally conductive cover are needed.
Flexible OLED devices are formed with flexible substrates of metal foils, plastic films, and the like, and offer certain advantages. These devices are lightweight, durable, and impact resistance. Their use in lighting applications and displays for cell phones, PDAs, portable computers, and so forth, is gaining wider acceptance. The flexible OLED devices are, however, subject to the same high temperature issues discussed above, in addition to the increased stresses in the light emitting materials resulting from bending or twisting the OLED devices. The solution proposed by the '518 publication discussed above is not suitable for flexible OLED devices because the increased thickness cathode and protective cover configuration only adds to the bending stresses and would likely induce cracking and delamination in the underlying layers.
Therefore, a need exists in the industry for an improved cathode structure and cathode protection layer particularly suited for flexible OLED devices that operate in high temperature environments.